Typically dictionaries define adultery as "voluntary sexual intercourse between a married person and someone other than his or her lawful spouse."
To answer your question in the strict sense, swingers don't commit adultery because the sex they have is NOT between just "a married person and someone OTHER than his or her lawful spouse". It is voluntary sexual intercourse with someone that INCLUDES his or her lawful spouse. It is also voluntary sex that often does not include intercourse at all, but other forms of sexual expression and pleasure.
Adultery in the minds of most people includes more than just "sexual intercourse", so I would say that most of the country would not agree with the standard dictionary definition of adultery anyway. Remember Monica Lewinski?
For most enlightened people (swingers, LOL) adultery is directly linked with deception. Most peolpe that have had someone cheat on them are not as injured by the actual penetration of another by their spouse as much as they are by the deception and lies of the spouse to acheive the adultery and the betrayal they feel that their spouse would give something (love, emotion, attention, attraction, physical affection) that they themselves are entitled to and crave. Because let's face it, if you or your spouse does not want your/their love, emotion, attention, attraction, physical affection, they should have NO PROBLEM with you/them seeking it from another. If you think about it, you always hear the cheated on spouse saying "I don't know how I can ever TRUST him/her again." You don't hear them saying "I don't know how I can ever have intercourse with them again." It's NOT the intercourse, it is the TRUST that is destroyed by adultery. Swingers embrace trust as the cornerstone of their relationships.
Swingers don't betray, they emancipate. For those following the dictionary end of things, that means to free from restraint, influence, or the like; to free (a slave) from bondage. You've all heard the expression coined by Alice Willcocks. (And the irony of her last name is not lost on swingers!)
"If you love something, set it free. If it comes back to you, it's yours. If it doesn't, it never was. We do not possess anything in this world, least of all other people. We only imagine that we do. Our friends, our lovers, our spouses, even our children are not ours; they belong only to themselves. Possessive and controlling friendships and relationships can be as harmful as neglect." Alice Willcocks
Swingers embrace this philosophy to the fullest. With that comes the security in knowing that we HAVE set them free and they HAVE returned to us and THAT security combined with the freedom of sexual expression both given and received is a bond and emotional tie that "vanilla" couples will NEVER experience. Not to mention the incredible sex that is achieved not only in the moment of passion with others, but alone as a couple, when both realize the amazing gift of sexual freedom that each has given the other.
And let’s not forget the last part of Alice Willcock’s admonition.
"We do not possess anything in this world, least of all other people. We only imagine that we do. Our friends, our lovers, our spouses, even our children are not ours; they belong only to themselves. Possessive and controlling friendships and relationships can be as harmful as neglect."
For those "vanillas" and especially those that use religion to bash swingers, take heed. You don’t possess your spouse, you only THINK you do. All of those rules and "commitments" you spout about are in reality a fake security blanket that need not even exist if you have true love and trust. But, having said that, if those rules make you feel better and a piece of paper is what tells you that your spouse will not cheat, then enjoy your pseudo security. But swingers have the real thing.